1. Field of the Invention
The present invention relates to a wire electric discharge machine for monitoring change of tension of wire by aged deterioration, lifetime, and the like.
2. Description of the Related Art
It is of critical importance to control tension of wire according to required value. When the tension is too weak, the wire is bent causing deterioration of machining precision. When the tension is too strong, breakage of the wire during machining is increased causing deterioration in operation rate of the machine. The tension of the wire is generally adjusted at the time of shipment, but long time use sometimes causes change of the tension value. In this case, malfunction as mentioned above is generated, to be a critical problem.
For this reason, some trials are performed to keep the tension of the wire regardless of deterioration of the machine. Japanese Patent Laid-Open No. 2003-266247 discloses a prior art to keep the tension to the wire to be constant. To quote from the Japanese Patent Laid-Open No. 2003-266247, a tension control signal is obtained by differential amplification of a tension detection signal and a tension command signal which is generated based on a tension set value and a tension control start signal. A signal obtained by differential amplification of a wire speed command signal and a speed detection signal is added to the tension control signal, and the result is given to a servo driver of a brake motor as an electric current command. An initial value of a wire electrode at start of tension control is set to be the tension command signal, and the value is controlled to reach the tension set value according to time width from the start of tension control and an increase pattern set in advance.
The tension is controlled to keep constant value based on the technique described above. Machining is immediately stopped in general, when feedback control fails by trouble of a tension generation system or a tension measurement system, or when the feedback control is expected to be failed.
Considering the above problem, in Japanese Patent Laid-Open No. 3-213215, a tension target value and a detected tension value are compared by a comparator to obtain deviation thereof, and alarm message is displayed when the deviation is beyond a tolerance range. Thus, a means for detecting failure to achieve target tension value by trouble of the tension generation device (a brake apparatus) or by deterioration caused by the end of a product lifetime, is provided.
However, in the technique disclosed in the Japanese Patent Laid-Open No. 3-213215, failure of the machine can not be determined before complete failure of the feedback control, since abnormality is detected by monitoring the deviation. This means that, the technique has a problem that defective rate of the processing is increased since the failure can not be detected in an early stage.
As an example, a case in which movement of a rotation part of the tension measurement device becomes slow by deterioration over time or the like, will be considered. As another example, a case in which slippage is generated in a feed roller, which is a component of the tension generation system, by frictional wear. When the feedback control of the tension is kept, a case in which the failure can not be determined is assumed to occur. The machine is in a state where the feedback control fails in the former case, while in a state where the feedback control itself has no abnormality in the latter case.